The present invention relates to a manufacturing method for an optical waveguide device used for the transmission of signal light in the field of, for example, optical communication.
FIGS. 5 and 6 show an optical waveguide device 21 such as an optical star coupler, which is used as an optical integrated circuit (optical IC) for optical communication, for example. As shown in FIG. 6, the device 21 has an embedded waveguide structure, and its refractive index distribution is of the step-index type. The device 21 comprises a substrate 22, formed of a quartz or silicon wafer, buffer layer 23 formed on the substrate 22, core 24, and upper clad layer 25. The buffer layer 23 and the upper clad layer 25 constitute a clad layer as it is called herein. The optical star coupler, as an example of the optical waveguide device 21 shown in FIG. 5, is used as a power branching for signal light. In this device 21, the signal light is inputted through one end 26 of the core 24 or an input end, which is situated on the left-hand side of FIG. 5, and is outputted through a plurality of other ends 27 of the core 24 or output ends, which are situated on the right-hand side of FIG. 5, for example.
Conventionally, the optical waveguide device 21 is manufactured in manufacturing processes shown in FIG. 7, for example. In a buffer forming process S11 and a core forming process S12, a buffer layer 23, consisting mainly of SiO.sub.2, and a core 24 are formed on the surface of a substrate 22 by a film-forming method, such as an FHD (flame hydrolysis deposition), CVD (chemical vapor deposition), or PVD (physical vapor deposition) method. In a patterning process S13, a specific waveguide pattern is formed on the surface of the core 24 with use of a photoresist, and is then etched by RIE (reactive ion etching) or some other method, whereupon the core 24 is shaped into a desired pattern. In a clad forming process S14, thereafter, the lower refractive index upper clad layer 25 is formed containing SiO.sub.2 as its main ingredient by the aforementioned film-forming method, such as the CVD or PVD method.
In a cutting process S15, the substrate and the like are cut into a given shape by means of a dicing machine, whereupon each individual optical waveguide device 21, such as the optical star coupler shown in FIG. 5, is obtained. An input-side end face 26a and an output-side end face 27a are individually polished to be finished into optically flat surfaces.
The refractive index of the core 24 must be made about 0.2% to 0.32% higher than those of the buffer layer 23 and the upper clad layer 25. To attain this, a doping agent for refractive index enhancement is introduced into the core 24 in the core forming process S12, or a doping agent for refractive index reduction is introduced into the buffer layer 23 and the upper clad layer 25 in the buffer forming process S11 or the clad forming process S14.
According to the FHD method, the speed of deposition of the film to be formed is high enough to form suitably the buffer layer 23, core 24, and upper clad layer 25 that are relatively thick. In forming these layers 23, 24 and 25 by the FHD method, material powder, consisting mainly of SiO.sub.2 and the like, is deposited on the substrate 22 and is then heated to a high temperature of 1,200.degree. C. to 1,500.degree. C. Thereupon, the material powder melts and forms a transparent glasslike structure. In this FHD method, however, the high temperature of 1,200.degree. C. or above is used, so that the doping agent may possibly diffuse, or SiO.sub.2, the main ingredient of the layers 23, 24 and 25, may flow. Thus, the layers 23, 24 and 25 cannot be easily formed into a desired shape.
In the cases of the CVD and PVD methods, on the other hand, the buffer layer 23, core 24, and upper clad layer 25 can be formed at a temperature (e.g., 500.degree. C. or thereabout) lower enough than in the FHD method. It is relatively easy, therefore, to regulate the thickness and shape of the layers 23, 24 and 25 with high accuracy.
However, the low-temperature film-forming method such as the CVD or PVD method involves the following problems. In the case where the clad layer 25 is formed on a plurality of cores 24a and 24b that have a rectangular cross section and are formed on the buffer layer 23, as shown in FIG. 6, the clad layer 25 sometimes may fail to fill the space between the cores 24a and 24b so that a void 28 is generated. If the space between the cores 24a and 24b is narrow (e.g., 5 .mu.m or less), in particular, the void 28 is generated frequently. If the void 28 is generated, a gap is formed between the clad layer 25 and parts of the respective peripheral surfaces of the cores 24a and 24b. The void 28 entails an increase in loss of the signal light when the light is transmitted through the optical waveguide device 21.
In the case where the layers 23, 24 and 25 are formed at a low temperature by the CVD or PVD method, they tend to be lowered in density and transparency. Thus, the optical waveguide device 21 formed by the low-temperature film-forming method, such as the CVD or PVD method, is subject to substantial scattering of the signal light.